Such a liquid collector often is called a “cup” even though it does not necessarily have a closed bottom. Another word often used for the liquid collector is “chamber” even though it is not closed on all sides.
Such a device is known in the art and described in details in U.S. Pat. No. 4,903,717. This patent shows each collector of each collector level being connected to a common exhaust. Each collector level thereby simultaneously serves as an exhaust level. Therefore while the common exhaust is turned on gas is sucked from the interior of the liquid collector by each exhaust level.
During processing it might happen that below the level of the spin-chuck the gas pressure is lower than ambient gas pressure. Consequently liquid which is ought to be spun in to a specific collector level might be partly sucked into the collector level below the selected collector level.
If a liquid X is brought to a wrong collector level in which a different liquid Y shall be collected liquid X will contaminate liquid Y. In the worst case liquids X and Y react with one another generating hazardous or flammable reaction products.
Another undesired result might happen if liquid Y is recycled in order to treat as many substrates as possible. A contamination of liquid Y with liquid X then might result in the destruction of the substrates treated thereafter. In this case another consequence could be a significant decrease of the shelf life of liquid Y.